Nickel base alloy



United States Patent ABSTRACT OF THE DISCLOSURE A wrought Ni-Cr-Co base alloy provides improved strength through a balance of the precipitation hardening elements Al, Ti and Cb and the solution strengthening elements M0 and W. A Cb-rich gamma prime phase preferentially is formed.

Disclosure This invention relates to nickel base alloys and, more particularly, to a wrought nickel base alloy having im- 3,403,059 Patented Sept. 24, 1968 columbium and titanium are particularly controlled to give a columbium rich gamma prime phase in the presence of other carefully balanced elements has heretofore not been recognized within the broad ranges previously reported. Furthermore, although certain nickel base alloys of this type specify the equivalence, particularly on an atomic basis, of the solution strengthening elements tungsten and molybdenum, it has been recognized in the type of alloy to which this invention relates that these two elements are, in fact, not equivalents. They must both be present 'within the range of the present invention in order to cooperate with the other elements, particularly columbium, titanium and aluminum, to provide an alloy of balanced high strength and ductility. As will be mentioned later, such elements as chromium and cobalt provide oxidation resistance and workability, respectively, and carbon has effect on ductility and de-oxidation.

Typical of the wide variety of alloy forms melted and tested in an evaluation of the alloy of the present invention are those presented in the following Table I.

. TABLE I [Composition (wt. percent); balance, Ni and incidental impurities] Example Al Ti Cb i 00 Cr Mo W O B V proved high strength from room temperature to about 1400 F.

The technology related to power producing apparatus continues to advance and continues to demand the development of alloys having higher strength at very high temperatures. Nevertheless, there is a need for improved high strength alloys which can operate up to about 1400 F. for use as lighter weight components such as forged turbine or compressor wheels or in sheet metal structures.

Therefore a principal object of the present invention is to provide an improved nickel base alloy particularly useful in the Wrought form as sheet or forging alloy and having improved high strength characteristics up to about 1400 F.

This and other objects and advantages will be more readily recognized from the following detailed description and examples which are meant to be exemplary of rather than any limitation on the scope of the present invention.

Briefly, the alloy of the present invention provided improved strength from an unique combination of the precipitation hardening elements aluminum, titanium and columbium and the solution strengthening elements molybdenum and tungsten. By weight, the alloy consists essentially of 33-42% Al; 20-28% Ti; 1.6-2.3%.Cb; 13- 17% Co; 13-16% Cr; 3-4% Mo; 56% W; 0.040.2% C; 0.0050.02% B with the balance nickel and incidental impurities.

The element columbium has been recognized and included 'with aluminum and titanium as a precipitation hardener and a gamma prime phase former. However, the unusual and improved high strength which can be achieved in aging heat treatment of an alloy when the aluminum,

These alloys were vacuum induction melted in approximately 15 lb. heats and cast into two /2" x 3" x 6" slab ingots per heat. These ingots were converted to 0.050"- 0.060" thick sheets by hot rolling. After rolling, the examples of Table I, with the exception of Examples 256, 412 and 413 were solution heat treated at 2100 F., air cooled and then aged for 16 hours at 1400 F. after which they were air cooled. Example 256 was solutioned at 2050 F. followed by air cooling and aging at 1400 F. for 16 hours. Examples 412 and 413 were solutioned at 2150" F. followed by air cooling and a double aging of 4 hours at 1650 F., air cooling and then 16 hours at 1400 F. followed by air cooling. Proper heat treatments were selected by first determining the gamma prime solution temperature in metallographic studies for each alloy. This solution temperature and appropriate aging treatments were used for property tests. Average values for the 1400" F. tensile testing is shown in the following Table 11.

TABLE II [1,400 I average tensile properties] Ultimate strength 0.2% yield Elongation, Example (1,000 p.s.i.) strength percent (1,000 psi.)

1 Brittle fracture before 0.02% Y.S.

A comparison of Examples 476, 481, 256 and 482 shows that with the other elements constant and within the range of the present invention, variation in the columbium content can make a significant and unexpected difference in the 1400 F. tensile properties. The differences in ultimate strength from this variation in a single element is readily noted and the production of a brittle alloy in Example 482 as shown by the addition of 4 weight percent columbium. This alloy form exhibited hot short cracking tendencies. Consequently it was diflicult to obtain usable sheet. However, there was enough sheet produced from this form for several tests. During testing, however, because of its brittleness, the Example 482 specimen fractured even before the 0.02% yield strength point could be reached, going directly to its ultimate strength point.

Comparison of the data in Table II for Examples 618 and 624 indicates that the addition of about 2% Cb to another system within the broad scope of this invention causes a significant increase in yield strength even though the Al and Ti contents are lower than the preferred form of the invention shown by Examples 481 and 256. However, as shown by Examples 496 and 499, the large improvement in yield strength does not occur with the addition of about 2% Cb to systems outside the scope of the present invention in which the Mo-W combination is different from the 3-4% Mo, 56% W range as used in Examples 256, 481 and 624. This point regarding the Mo-W range is further illustrated in Table III in which several different variations of Al, Ti and Ch are used with a 6% Mo3% W type alloy compared with Example 624.

In a subsequent program to further evaluate the alloy of the present invention, an Example 258 was melted consisting essentially of 4.2% Al, 2.8% Ti, 2.2% Cb, 15.0% C0, 15.0% Cr, 9.0% Mo, 0% W, 0.12% C, 0.009% B with the balance nickel and incidental impurities. During vacuum melting of this heat, and during a remelting operation, the electrode was reported to explode. Then in subsequent forging performed at 2000 F., the 9% Mo composition yielded only about due to cracking. Subsequently this Example 258 could not be rolled into bar even after a second evaluation. This indicates that the 9% Mo chemistry is responsible for the poor hot working characteristics of the alloy otherwise within the range of the present invention.

Thus as shown by the above tables, it has been recognized that there is a particular balance of columbium, aluminum and titanium to result in high strength which would not result from a separate consideration of columbium and of aluminum and titanium. The absence of columbium results in significantly lower properties, particularly with regard to the 0.2% yield strength. A columbium level of 4% is too high and results in a very brittle alloy which fractured before it reached the 0.02% yield strength. In general, it has been found that both the improved ductility and strength of the alloy of the present invention can be maintained within the range of about l.62.3% columbium. Below about 1.6% Cb lower strength results. Above about 2.3% the alloy becomes brittle. It has been found that with this particular amount of columbium, the aluminum should be maintained in the range of about 33-42% and the titanium should be maintained in the range of about 2.02.8% in order to TABLE III Composition (wt. percent); other, about 15 Co, 1,400 F. Tensile 15 Cr, .1 C, .01 B, Bal. Ni Example Al Ti Mo W Cb UTS 0.2 YS E1 (K s.i.) (K s.i.) (percent) It is to be noted that Example 617, which does not include Cb, has properties not appreciably different from those which do include a 6% Mo3% W type system.

As shown in Table II by Example 622, compared with 624, it is to be noted that the substitution of approximately the same atomic percent of vanadium for columbium results in a decrease rather than an increase in tensile strength properties.

Examples 412 and 413 in Table II are typical of a number of alloy forms which were melted and tested to show the nonequivalency of molybdenum and tungsten in the type of alloy to which the present invention relates. Typical of the other results obtained, the substitution of tungsten for part of the molybdenum results in greater strength and improved ductility than might be expected from such a substitution. The tungsten addition contributes more to strength than does molybdenum alone. It is to be noted that the total atomic percent of the solution strengtheners in Example 413 is less than that of 412 because the molecular weight of tungsten is approximately twice that of molybdenum. Yet the strength and ductility of Example 413 is greater than that of 412. Because of the generally lower amounts of Al and Ti in the alloy of this invention, tungsten is required along with molybdenum to improve the overall strength of the alloy.

control the phases formed from titanium, aluminum and columbium in the presence of up to about 0.2% carbon.

With regard to the phases formed, during the solution heat treatment there is a solid solution including MC type carbides (with the M being primarily titanium and columbium) plus an excess of titanium, aluminum and columbium in solution. During the aging processing, the aluminum, titanium and columbium form a columbiumrich gamma prime phase. However, the formation of too much gamma prime can result in a brittle though strong alloy. Too little gamma prime results in a weak though ductile alloy. Therefore, the particular balance of aluminum, titanium and columbium in the alloy of the present invention controls the formation of gamma prime phase in the presence of the proper amount of carbon to result in an alloy of improved combination of strength and ductility.

The element carbon can be included in the range of about 0.040.2 weight percent. It was observed that alloys having a very low carbon content tended to have very low 1400 F. tensile ductilities and rupture life. These poor properties are attributed principally to low carbon content in heats in which the carbon was purposely maintained at as low a level as possible. Subsequent heats melted at carbon levels of for example, 0.080.1%, had

5 improved tensile ductility and rupture properties. This improvement is attributed to (1) better de-oxidation of the melt due to carbon de-oxidation and (2) excess car- 6 The following Table IV shows the composition of larger heats of examples melted within the range of the present invention.

TABLE IV [Composition (wt. percent) balance Ni] Example Al Ti Ch Cr Mo W G B 1 pound vacuum induction heat. 2 50 pound vacuum induction heat. 3 1,000 pound vacuum induction pl us vacuum consumable electrode remelt heat.

The following Table V shows the tensile properties for the heats of T able IV.

TABLE V.TENSILE PROPERTIES bon in the alloy that precipitates and forms agglomerated particles within grain boundaries which destroy the continuity of embrittling carbide films. These carbide films (MC and M C tend to form even in the very low carbon heats during welding and during aging. Since it is not feasible to obtain heats with sufficiently low carbon to completely prevent carbide precipitation, it was found that it is better to have enough carbon present that will precipitate and grow to a less harmful morphology.

It has been found that cobalt should be included in the alloy of the present invention in the range of about 13- 17% for workability. As was shown by a number of alloys, an increased cobalt content results in better workability but the tensile ductility decreases. For example, in alloy forms 314, 315 and 316 tested, having cobalt contents of 15, 20 and respectively, with 4.14.3% Al, 2.4-2.6% Ti, 12% Cr, 7.47.6% Mo, .01.03% C with the balance Ni and impurities, the tensile ductility at 1400 F., decreased from 7.6% to 5.2% to 3.6% respectively. This is typical of addition of cobalt to the precipitation hardening type nickel base alloys. Thus the embrittlement of the alloys during aging tended to be higher in the heats melted with higher cobalt (with heats up to as high as 25% were investigated). This may also be related to carbide film embrittlement since it has been shown that higher cobalt content tends to increase the solubility for carbon. Thus a high cobalt heat for the given carbon content would behave much like a lower carbon heat having a lower cobalt content. Since higher cobalt levels in these types of alloys improve hot working characteristics, it is desirable that cobalt be used at as high a level as possible. Thus the alloy of the present invention has established that with the other elements present, the cobalt range should be maintained at about 13-17 weight percent.

Chromium is included for the purpose of imparting oxidation resistance to the alloy. However, it was shown that the inclusion of 12.5% Cr resulted in an alloy which showed selected oxidation at the point of failure during an 1800 F. rupture test. The inclusion of percentages higher than about 16% results in greater formation of sigma phase and can lead to processing problems. Therefore, the chromium range of the present invention was established at about 1316 weight percent.

Thus the alloy of the present invention provides an improved combination of strength and ductility for application up to about 1400 F. by an unexpected selection and balance of elements. Because of the improved strength and ductility characteristics, lighter weight components fulfilling strength and ductility requirements can be produced from this alloy.

Although the present invention has been described in connection with specific examples, these are meant to be typical of rather than any limitation on the present invention the scope of which is defined by the appended claims.

What is claimed is:

1. A wrought nickel base alloy of improved strength up to 1400 F. consisting essentially of, by weight, 3,3- 4.2% Al; 2.02.8% Ti; 1.62.3% Cb; 13-17% Co; 13* 16% Cr; 3-4% Mo; 5-6% W; 0.04-0.2% C; 0.005-0.02% B with the balance nickel and incidental impurities to form preferentially a columbiurn-rich gamma prime phase.

2. A wrought nickel base alloy of improved strength up to 1400 F. consisting essentialy of, by weight, 3.8- 4.2% A1; 2.42.8% Ti; 1.6-2.3% Cb; 13-17% Co; 13- 16% Cr; 34% Mo; 56% W; 0.04-0.2% C; 0.005- 0.02% B with the balance nickel and incidental impurities to form preferentially a colurnbium-rich gamma prime phase.

3. A wrought nickel base alloy of improved strength up to 1400 F. consisting essentially of, by weight, about 4.0% A1; about 2.5% Ti; about 2.0% Cb; about 15% Co; about 15% Cr; about 3.5% Mo; about 5.5% W; about 0.1% C; about 0.01% B with the balance essentially nickel and incidental impurities to form pereferentially a columbium-rich gamma prime phase.

References Cited UNITED STATES PATENTS 3.151,981 10/1964 Smith et al -171 3,155,501 11/1964 Kaufman et al. 75-171 3,164,465 1/ 1965 Thielemann 75171 3,166,412 1/1965 Bieber 75-171 RICHARD O. DEAN, Primary Examiner. 

